Aircraft fuel cell system without the use of a buffer battery

ABSTRACT

In one or more embodiments of the novel aircraft fuel cell system without the use of a buffer battery, the fuel cell and compressor would be sized sufficiently larger for the intended application, allowing the compressor to change speeds much faster. This in turn would allow power outputs to change much quicker. If power outputs can change as quickly as the application dictates, then a buffer battery is not necessary. In one or more embodiments, because the system is mostly electronically controlled, software can be written to protect the fuel cell from instantaneous power pikes. If a large power output is suddenly requested of the fuel cell, the software can smooth out the demand curve to provide an easier load profile to follow.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This regular U.S. patent application relies upon and claims the benefit of priority from U.S. provisional patent application No. 62/808,569, entitled “AIRCRAFT FUEL CELL SYSTEM WITHOUT THE USE OF A BUFFER BATTERY,” filed on Feb. 21, 2019, which is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The disclosed embodiments relate in general to clean energy based air transportation systems technology, and, more specifically, to an aircraft fuel cell system without the use of a buffer battery.

Description of the Related Art

Conventional hydrogen fuel cell technology mixes pure hydrogen gas and oxygen found in the atmosphere to create an electrical charge across a catalytic membrane, commonly referred to as a cell. By combining multiple cells in series, a fuel cell stack is created, and thus the output voltage of the system can be increased. Secondly, the current that can be extracted from the fuel cell stack is dependent on the rate of the chemical reaction between the hydrogen and oxygen. Increasing the amounts of these reactants yields an increasing amount of current within operational limits. Since the volume of a fuel cell is fixed by its physical dimensions, the two reactants must be pressurized to increase their masses per given volume.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification exemplify the embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the inventive technique. Specifically:

FIG. 1 illustrates aircraft fuel cell system architecture without the use of a buffer battery.

FIG. 2 illustrates feed forward load prediction algorithm control loop.

FIG. 3 illustrates electronically controlled air compressor for quick response.

DETAILED DESCRIPTION

In the following detailed description, reference will be made to the accompanying drawing(s), in which identical functional elements are designated with like numerals. The aforementioned accompanying drawings show by way of illustration, and not by way of limitation, specific embodiments and implementations consistent with principles of the present invention. These implementations are described in sufficient detail to enable those skilled in the art to practice the invention and it is to be understood that other implementations may be utilized and that structural changes and/or substitutions of various elements may be made without departing from the scope and spirit of present invention. The following detailed description is, therefore, not to be construed in a limited sense.

The pure hydrogen gas supply is commonly pumped into and stored in high pressure tanks at roughly 350 bar to 700 bar. The oxygen is sourced from the surrounding ambient air, containing roughly 21% oxygen, and thus must be pressurized by a device before entering the fuel cell stack to achieve high volumetric flow rates and high overall levels of power density for the fuel cell system. The device responsible for pressurizing and creating the necessary air flow is usually an air compressor. These air compressors come in two main varieties: mechanical compressors coupled to a shaft, or electronically controlled compressors.

Since the hydrogen supply is already in pressurized form, it is traditional to throttle the power output of the fuel cell by modifying the amount of air being fed into the fuel cell. This is done by speeding up or slowing down the the air compressor as required by the application. Because the air compressor is a physical object, there are limits to how quickly it can be sped up or slowed down, thus affecting the response time of the fuel cell's power delivery. Generally, the oxygen supply (provided by compressed air) is the limiting factor of fuel cell power output.

Secondly, as ambient air contains mostly nitrogen (78%), there are other chemical reactions and processes that take place within the fuel cell. These other processes lead to inefficiencies in the system, and thus do not allow the fuel cell to operate in a linear power output, nor without a minimum power requirement.

The minimum operating power requirement for a fuel cell stack is centered around the humidity level within the fuel cell stack. Since water vapor is a product of the chemical reaction between hydrogen and oxygen, controlling the fuel cell is as much about getting the water out, as it is getting the reactant gases in. The management of this water vapor is extremely important, as too much water vapor on each cell will reduce surface area available for future chemical reactions to occur, and thus lowering the power output of the stack; and too little water vapor on each cell will increase the available surface area for the reaction, increasing the reaction rate, and potentially causing the fuel cell to heat up and oxidize the membrane on which the reactions occur. This oxidation is irreversible, and will permanently damage the fuel cell's ability to produce its rated power output.

This minimum power requirement is a challenge for aviation use cases, as an aircraft's load profile is dramatically different from common fuel cell applications. Primarily, as the passengers board, the system is started up, and the plane taxis to the runway in preparation for take-off, the plane uses very little power, most-likely below the manufacturer's minimum power requirement for safe operation.

Currently, the most common implementation of a fuel cell system requires a buffer battery to physically drive the load/application, leaving the fuel cell system to only charge the battery as needed. If power required by the application is below the manufacturer's minimum power requirements, the vehicle uses the buffer battery to propel itself. Once the system deems the battery must be charged, the fuel cell kicks on and charges the battery. This creates a smooth and known power requirement for the fuel cell above its minimum operating power threshold, while the battery tailors to the dynamic nature of the load.

In the proposed system, there are quite a few inventive techniques that allow the successful operation and deployment of fuel cell technology without the use of a buffer battery, with no detrimental effects to performance and the application.

1. Actively Managing Fuel Cell Load Demand—Below Minimum Power

The fuel cell system is designed to operate in a very narrow temperature range. At power levels above the manufacturer's minimum power output, the fuel cell will continue to heat itself in the process of generating power output and regulate its own temperature. In the case when the power generation is below the manufacturer's minimum power and the fuel cell cannot raise its own temperature to the specified range, ZeroAvia proposes:

To add a sizeable load to the system via a resistive heater circuit(s) in the fuel cell's coolant circuit(s).

This heater circuit(s) will be combined in parallel with the load and fuel cell output.

This strategy will accomplish two things: simultaneously raise the load power drawn from the fuel cell, and heat up the coolant circuit(s) to the correct operating temperatures.

While a traditional system may use a battery to operate below minimum power requirement levels, ZeroAvia proposes this method of increasing the load power.

2. Actively Managing Fuel Cell Load Demand—Above Maximum Power

The power output of a fuel cell is limited by the reaction rate, and as such, the oxygen supply is usually the limiting factor of traditional systems. If oxygen supply is taken care, and there are more claims below on what can be done to accomplish this, then the next limiting factor for fuel cell power output is thermal management and keeping the fuel cell stack within the correct operating temperature range. ZeroAvia proposes the following:

Use an active cooling system, controlled and monitored by the fuel cell system to actively cool the fuel cell stack to allow operation past the designed maximum power level.

This can be done with traditional cooling circuits consisting of coolant pumps, temperature sensors, heat exchangers, and valves to direct and confirm the direction of flow throughout the system.

While a traditional system may use a battery to operate short intervals above the fuel cell's maximum output levels, ZeroAvia proposes this method to allow the safe operation of the fuel cell without the use of a buffer battery.

3. Lowering the Stack Temperature to Facilitate a Lower Minimum Operating Power

The fuel cell stack is an actively cooled component, with specifically targeted operating temperatures. These temperatures promote ideal reaction rates at each cell membrane, thus promoting peak power outputs. While the manufacturer of the fuel cell designs it for a specific maximum power output, the idea is to operate the fuel cell at a lower core temperature when the power required by the application is below the minimum power threshold of the fuel cell stack. This provides the following benefits:

Lower stack core temperature creates a higher temperature delta as the fuel cell operates between cell membrane temperature and reaction temperature

This higher temperature delta facilitates the formation of more condensation on the cell membranes

More condensation on each cell cools the membrane and prevents the damaging oxidation from occurring

Lack of oxidation on the cells promotes low power applications, thus not requiring the buffer battery

4. Feed Forward Load Prediction Algorithm

During normal operation of the fuel cell, the response time of the fuel cell power output compared to the application requirements is almost exclusively due to the physical ramp up of the air compressor. At or close to maximum power levels, the fuel cell also begins to encounter a thermal limit to its power output. The idea is to use multiple inputs from the pilot and control system to generate a future load calculating algorithm in preparation of future conditions and power requirements and to adjust the air supply and active cooling mechanisms built into the fuel cell control system

Using a series of pilot operated inputs along with calculations from the plane's avionics and control system the compressor speed will be actuated on future power requirements, rather than instantaneous power requirements.

This ensures that the fuel cell has the correct amount of airflow at all times, regardless of external factors that are not present instantaneously

This will decrease power delivery response time from the fuel cell, contributing to an overall safer operation of the fuel cell system

If the cooling circuits are also ramped up in preparation for high power output, then safer and more reliable operation of the fuel cell will occur.

This anticipatory calculation is the feed forward portion of the algorithm.

5. Electronically Controlled Air Compressor for Quick Response

In order for this fuel cell system to operate without a buffer battery, the power delivery must be smooth, controllable, and predictable as the load requirements change. Since the fuel cell system is almost exclusively throttled through the flow of oxygen into the fuel cell stack, the supply of oxygen must also be equally controllable. In order to accomplish this, the air compressor supplying the oxygen must be controllable independent of the load. There are two methods to achieve this:

This is achieved by using an electronically controllable air compressor, which is fed input signals from an external device. This external device will be the flight computer onboard the aircraft.

This can be achieved by using a shaft-driven mechanical air compressor, using the drivetrain for its input power. In order to control the flow and pressure of air into the fuel cell system at various engine speeds, the compressor will have either an electronically controlled valve or spring actuated wastegate that bleed unnecessary air away.

6. Oversized Air Compressor for Quick Response

Generally, the air compressors are sized to provide the fuel cell's peak output power at their fastest speeds/highest flow rates. Since there is a certain time lag for the air compressor to speed up and slow down as power demands vary, the air compressor can be oversized to compensate for this lag. With a larger air compressor in the system, the fuel cell will be able to meet its air requirements sooner, as it does not need it to come to full speed. If the air requirements are met sooner, then the use case for a buffer battery is eliminated.

7. Paralleled Air Compressors for Quick Response

Conversely to the previous inventive idea, if a larger air compressor is not feasible due to packaging constraints, several air compressors plumbed in parallel may be used to meet the fuel cell's air requirements. These air compressors may vary in size, or be staged in a compounding setup to better fit the packaging and air requirements of the application. Smaller air compressors will have less inertia to overcome and therefore are able to change their flow rates quicker. If the air requirements are met sooner, then the use case for a buffer battery is eliminated.

8. Starting the Fuel Cell System without an Onboard High Voltage Source

In order for fuel cell systems to start creating power, air flow must first be introduced. As discussed previously, the air flow traditionally comes from an onboard electric air compressor. These air compressors are generally high voltage items, and thus require a high voltage source to begin operating. This high voltage source is usually the buffer battery. Without a buffer battery ZeroAvia proposes the following methods of starting the fuel cell system:

Because only a small amount of air flow is required to begin the fuel cell system, a small low voltage air compressor can be designed into the system that operates at the traditional 24V aviation voltage level. This compressor would be sized so that it could provide enough air flow to start the main compressor(s) in the fuel cell system from the fuel cell output power at any point in the plane's flight plan, primarily used for ground start operation, and emergency in-flight restart operation

A ground start procedure using a high voltage battery can be implemented to start the fuel cell process. From that point on, the fuel cell and air compressors would be self sufficient. If a failure occurred during flight, and the system needed to be restarted, a valve can be built into the plane and fuel cell system to utilize a “Ram-Air” effect using the plane's forward motion to provide the necessary airflow to restart the fuel cell system.

The fuel cell system can also carry an emergency supply of compressed air that can be used to start the fuel cell system in the event of an emergency. The compressed air can be sized to allow for several restart attempts, and will provide ample air to start the fuel cell system so it can then start the onboard high voltage air compressor.

The motor(s) in an electric drivetrain free-wheel when power is not applied. If an in-air restart needs to occur, and a mechanical air compressor is being utilized, rather than an electric air compressor, then the spinning of the propeller attached to the free-wheeling motors should be enough energy to spin the air compressor and generate the necessary air flow through the fuel cell system to power up the motor(s) and begin normal operation again.

FIG. 1 illustrates overall architectures—traditional and proposed Systems:

101—Fuel Cell: Provides electric power to inverter.

102—Air Compressor: Provides the necessary Air Flow to the Fuel Cell.

103—DC/DC Converter: Conditions the Fuel Cell Voltage Output to Charge the Battery.

104—Battery: In conventional systems, this is used to fill gaps where the Fuel Cell(s) lack power.

105—Inverter: Converts DC electricity from the fuel cell into AC electricity for the motor.

106—Motor: Generates torque to spin the propulsor's shaft.

FIG. 2 illustrates Feed Forward Load Prediction Algorithm Control Loop.

FIG. 3 illustrates Electronically Controlled Air Compressor(s) For Quick Response.

Figure A shows a typical system configuration with a single oversized air compressor.

Figure B shows a system configuration utilizing ‘n’ number of air compressors.

Finally, it should be understood that processes and techniques described herein are not inherently related to any particular apparatus and may be implemented by any suitable combination of components. Further, various types of general purpose devices may be used in accordance with the teachings described herein. It may also prove advantageous to construct specialized apparatus to perform the method steps described herein. The present invention has been described in relation to particular examples, which are intended in all respects to be illustrative rather than restrictive.

Moreover, other implementations of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. Various aspects and/or components of the described embodiments may be used singly or in any combination in aircraft power plants. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. 

What is claimed is:
 1. An aircraft fuel cell system without the use of a buffer battery, the system comprising an oversized electronically controlled compressor that can directly and quickly control output power. 